You can't have a flatter frequency response than the Clip, since it is already flat (no matter what phone it's paired with). Never tested a Fuze, but what I remember from over on the Rockbox site, it is exactly the same.

They all sound the same, more or less - the O2 hisses the least with low impedance IEMs and has slightly less bass rolloff, so that would make it the best sounding one. Of course it is also the worst when it comes to usability, battery life, and so on.